Background: Legumes are outstanding sources of macronutrients, micronutrients, phytochemicals, as well as antinutritional factors. These components present a complex system enabling interactions with different components within food matrices. The interactions result in insoluble complexes with reduced bioaccessibility of nutrients. The development of appropriate preparation technologies for use at the household and village-level become so imperative to facilitate processing and dietary availability of beans. Aim of the Study: This study aimed to evaluate the effect of thermal processing on the chemical contents of unfermented and fermented red kidney beans (Phaseolus vulgaris) and the effects of the resulting changes on the in vitro protein digestibility. This will enhance food security and reduction in malnutrition. Methodology: Unfermented and fermented P. vulgaris were boiled using ordinary cooking pot and a pressure pot and the chemical contents were evaluated by standard methods. In vitro protein digestibility was carried out by pepsin digestion. Results: Fermentation resulted in a decrease in the traditional cooking time in the ordinary cooking pot by 40.32%. The protein content of the fermented sample increased by about 7%. The in vitro protein digestibility value was increased by more than 30% with greater percentage evident in fermented samples. Sulphur containing amino acids, methionine and cystein were the limiting amino acids but their contents appreciated by 6.64% and 10.92% respectively after fermentation. Total ash, crude fibre, crude fat contents of P. vulgaris were all affected by more than 20% during the open fermentation and cooking of unfermented beans. The antinutritional factors of P. vulgaris decreased most in boiled fermented samples compared with the other processing methods. There was overall improvement in the In vitro protein digestibility, reduction of cooking time and antinutritional factors when P. vulgaris was fermented and cooked. Conclusion: The outcome of the reserach justifies the fact that combining both fermentation and cooking results in the overall improvement in the nutritional value of P. vulgaris as against cooking without fermentation.